The present invention relates in general to electromagnetic (EM) geophysical method of determining the electrical conductivity (or resistivity) of earth formations penetrated by a well bore. The method can be applied for studying the underground geological structures in mineral, hydrocarbons and groundwater exploration, for example, for determining the low conductive zones associated with oil and gas reservoirs.
Formation conductivity (or resistivity) determination from a wellbore is one of the oldest geophysical techniques to be applied in oil and gas exploration and production. The idea and principles of a geophysical method of EM induction well logging were introduced in the pioneering paper by H. G. Doll, Introduction to Induction Logging of Wells Drilled with Oil Based Mud, Journal of Petroleum Technology, vol. 1, p. 148, Society of Petroleum Engineers, Richardson Tex. (1949). Over the decades, many modifications and improvements have been made in this area. These modifications can be found in the numerous patents, for example, U.S. Pat. No. 5,452,762 issued to Beard et al., U.S. Pat. No. 5,781,436 issued to Forgang et al., U.S. Pat. No. 6,556,016 B2 issued to Gao et al.
In conventional induction well logging, an instrument having transmitter coils and receiver coils substantially parallel to the borehole axis is lowered into the borehole and measures the magnetic field generated by the eddy currents induced in the earth formations. Certain earth formations containing petroleum and permitting the petroleum to flow through the rock comprising the formation have certain physical characteristics well known in the art. For example, hydrocarbon-bearing formations are typically very resistive, while water-bearing formations are typically very conductive. A limitation to the EM induction well logging method known in the art is that the response of the typical EM induction logging instrument is largely dependent on the conductive layers, while the effect of the nonconductive hydrocarbon-bearing reservoirs can be masked by the influence of the conductive layers.
At the same time, it is well known that the rock formations surrounding the borehole may be anisotropic with regard to the conduction of electrical currents. The phenomenon of electrical anisotropy is generally a consequence of either microscopic or macroscopic geometry, or a combination thereof. The reservoir anisotropy was identified as the major factor affecting the induction well logging data by Klein et al. (1997). Basically, the authors of the cited paper found that in a hydrocarbon-bearing reservoir at least two separate components of resistivity can influence the induction instrument: the resistivity measured with current flowing parallel to the bedding planes, which is called the transverse or horizontal resistivity ρh, and whose inverse is the horizontal conductivity, σh=1/ρh; the resistivity measured with a current flowing perpendicular to the bedding plane, which is called the longitudinal or vertical resistivity, ρν, and whose inverse is the vertical conductivity σν=1/ρν. The ration, λ=√{square root over (σh/σν)}, is called the anisotropy coefficient of the medium. Klein et al. (1997) found that the anisotropy coefficient for a hydrocarbon-bearing reservoir could rich the value up to 100. In this situation the conventional induction well logging tool, oriented substantially perpendicular to the bedding planes, is sensitive to the horizontal resistivity of the formation only, while the instrument oriented at an angle with the bedding planes reads the apparent conductivity response, σa, which can be any value between √{square root over (σhσν)} and σh.
One solution to this limitation known in the art has emerged in recent years. It is based on introducing transverse transmitter coils and receiver coils, with magnetic moments oriented perpendicular to the borehole axis. The idea of transverse induction coil measurements appeared first in the former Soviet Union (Eidman, 1970; Kaufman and Kaganskii, 1972; Tabarovsky et al., 1976). It has received further development in many inventions. Forgang et al. (U.S. Pat. No. 5,781,436) introduced method and apparatus for transverse electromagnetic induction well logging. Wu (U.S. Pat. No. 5,886,526) described a method of determining anisotropic properties of anisotropic earth formations using a multi-spacing induction tool. Hagiwara (U.S. Pat. No. 5,966,013) disclosed a method of determining certain anisotropic properties of formation. Gao et al. (U.S. Pat. No. 6,393,364) introduced an iterative method for determining the horizontal and vertical resistivity. Gupta et al. (U.S. Pat. No. 5,999,883) considered a triad induction tool. Gao et al. (U.S. Pat. No. 6,556,016 B2) introduced an induction method for determining the dip angle of an anisotropic earth formation surrounding a wellbore. In yet another development Zhdanov, Kennedy, and Peksen, in the article entitled “Foundations of the tensor induction well logging,” developed the basic physical and mathematical principles of electromagnetic tensor induction well logging in anisotropic formation.
Another limitation of the induction well logging data, known in the art, is the significant borehole effect that can distort the response of the formation surrounding the borehole. This effect has to be eliminated from the data, otherwise interpretation would be erroneous. The methods for the reduction of the wellbore effect were developed by Tabarovsky and Epov (1972). They introduced a “frequency focusing” technique based on induction voltage measurements at two frequencies and combining the observed signal in a manner so that the effects of eddy currents flowing within the wellbore can be substantially eliminated from the final result. This “dual frequency” signal is widely used in interpretation of conventional induction logging data and in transverse induction coil measurements as well (Forgang et al., U.S. Pat. No. 5,781,436).
The above references are incorporated herein by reference.
The foregoing attempts to determine vertical and horizontal resistivity around a deviated borehole and/or the dip angle of the formation met with varying degrees of success. However, there remains a need for improved methods and apparatus for high resolution hydrocarbon-bearing reservoir identification and characterization. A new technique is therefore needed.